Process of coating the exterior surface of a fluorescent lamp envelope with silicone resins



United States Patent PROCESS OF COATING Tim EXTERIOR SURFACE OF AFLUORESCENT LAMP ENVELOPE WITH SILICGNE RESINS Robert F. Quirk, Danvers,Mass, assign'or to-Sylvama Electric Products inc, a corporation ofMassachusetts No Drawing. Filed Mar. 16, 1960, Ser. No. 15,263

' 3 Claims. (Cl. l1'7--94) This invention relates :to a novelcomposition for use in coating the external surface of electricdischarge type lamps, particularly fluorescent lamps. Specifically theinvention relates to the art of depositing a water-repellent film on theexternal surface of a rapid start type of lamp.

It has been determined that external surface moisture tends to cause aleakage of electrons through the length of the glass envelope and it isthus desirable to reduce the condensation of this moisture. Suchreduction is particularly desirable in the rapid start type of lampwhere leakage tends to prevent starting of the lamp. It has been foundthat even high atmospheric humidity will cause this condensation and theensuing detrimental results.

It is known that the leakage could be reduced by coating the externalsurface of the glass envelope with a water repellent material.Generally, a silicone or silane has been used for this purpose.

In general, the prior art has utilized either of two procedures toeliminate the problem of condensed moisture. The first procedureinvolves reacting a silane, for example methyltrichlorosilane or ethyltrichlorosilane, in the vapor phase with a source of hydroxyl groups orthe silane has been reacted with absorbed water on the glass surface.The second procedure involves applying a thin film of polysiloxane, suchas dimethylpolysiloxane, to the glass surface from a solvent batch. Inboth of these procedures the glass surface was made more waterrepellent.

The first procedure has the disadvantage that one of the products of thechemical reaction involved is hydrogen chloride, a very corrosive gas.Lamps to which a water-repellent surface coating is applied by thismethod are normally processed in an area away from lamp producingmachinery so that the corrosive hydrogen chloride can be effectivelyremoved from the atmosphere without coming in contact with themachinery. Where high speed fluorescent lamp producing machinery isinvolved,

this procedure results in excessive handling of the lamps I and iseconomically unfeasible. Further, if machinery for applying the waterrepellent film to the glass surface were integrated with the high speedlamp producing machinery, elaborate equipment would be required toeffectively eliminate the contact of the corrosive hydrogen chloride gaswith the exposed metal parts of the lamp producing machinery.

The second procedure for applying the water repellent film to the glassenvelope can be integrated with high speed lamp producing machinery;however, the film obtained is degraded with time to such an extent thatmuch of the water repellency is lost prior to the failure of the lamp.The degradation is probably caused by autoxidation of the silicone film.

It is the object of my invention to coatv the external surface of glassenvelopes with a mixture of silicones which will maintain a high degreeof water repellency over the effective life of the lamp.

It is a further object of my 'nvention to provide a method of depositingthis film which is amenable to high speed fluorescent lamp productionequipment by using a fast drying solvent and by employing chemicalswhich are not corrosive or do not liberate corrosive gases as alay-product.

It is a further object of this invention to provide a water repellentfilm for use on the external surface of the 3,102,049 Patented Aug. 27,1963 ice , these ingredients a highlywater repellent film to the glasssurface which maintains the water repellency for a longer period oftimethan is possible with a film consisting of only a polysiloxane.

Without limiting the claims the following example is cited forillustrative purposes.

To 7.57 liters of isopropyl acetate, add 200 mls. ofdimethylpolysiloxane and 150 this of silicone resin. The mixture shouldbe stirred untilall of the dimethylpolysil-oxane and silicone resin isdissolved. This solution should then be stirred slowly into 182 litersof isopropyl alcohol and the agitation continued until the resultingmixture is completely and uniformly dispersed.

As a source of the dimethylp olysiloxane, it is preferred to use DowCorning DC 200 fluid. This fluid comes in various viscosity grades andalthough the 350' centistokes grade is preferred, viscosities from 20centistokes to 12,500 may be used successhilly. Dow Corning DC 200 fluidis produced by the Dow Corning Corporation and is composed ofdimethylsi'loxane units. The viscosity is a function of the length ofthe molecular chains, increasing with increasing molecular chain length.The preferred 350 centistoke viscosity grade has a freezing point of -50C. and a boiling point at less than 2 mm. Hg of 200 C. The specificgravity at 25 C./25 C. is 0.972 and the resin has a refractive index at25 C. of 1.4032.

The silicone resin that is preferred is the Dow Corning Silicone ResinZ920 manufactured by the Dow Corning Corporation. This silicone resin ispartially alkoxylated resin with the alkoxyl groups predominantlymethoxyl. Methods generally available for preparing the Z920 resininclude reacting CH Cl and Si in the presence of a copper catalyst attemperatures of 200 to 500 C. and separating from the reaction productall of the by-products having a boiling point above that of (CH SiCl Theby-products are then allcoxylated in an amount equivalent to at least 30molar percent of the hydrolyzable chloride present in the by-productmixture with a primary or secondary alkyl monohydric alcohol having from1 to 4 carbon atoms. The allcoxylated product is then hydrolyzed bycontact with water in quantities at least equivalent to the molardifference between the hydrolyzable chloride in the original mixture ofthe by-products and the amount of alcohol used, but insufficient tohydrolyze more than 70 molar percent of the total hydrolyzable groupspresent. Hydrolyzation with these quantities of water produces apartially alkoxylated or-ganopolysiloxane which is 20 to 50 percent byweight alkoxyl. When desired to extend the coverage of the resin, it maybe diluted with conventional solvents such as xylene or possibly tolueneor mineral spirits. When using xylene, the diluted mixture shouldcontain about 15 percent by weight solvent. Z920 has a viscosity at 25C. of 10 to cent-istokes, a specific gravity at'25 C. of 1.05 to 1.15and the color of light straw.

The above described coating solution is utilized by placing a suitableportion in a batch and the fluorescent lamps to which the waterrepellent film is to be applied are rolled through the solution in sucha way as to cause the lamps to make one complete revolution, therebycovering the entire external surface of the glass envelope. Guide barsare placed in the solution at a depth of about A; inch below the liquidso that the ends of the lamp and those parts of the lamp which will beemployed in making electrical contacts will not be coated.

The degree of water repellency of various compositions was measured byascertaining the contact angle between a glass surface and a drop ofdistilled water placed thereon. This measurement is easily made with aprotractor.

The test procedure employed in these tests was as follows. The glassused was soda-lime glass, the ingredients comprising approximately 73.6%SiO, 16% Na O, .6% K 0, 1.0% A1 5.2% CaO and 3.6% MgO, in 6 to 7 inchlengths. The glass, after sectioning, was washed in Warm tap water toremove surface dirt and then air dried. Silicone solutions were preparedand coated by dipping. The samples were then air dried in mildly warmair while standing on end. The water drop contact angle was measured.The samples were irradiated with ultraviolet light (quartz transmissionregion) from a high pressure mercury arc. The samples were arrangedparallel to the quartz tube and about 6 inches from it. Sampletemperatures were several centigrade degrees higher than ambient.Contact angles were measured after various exposure times to determinethe rate of degradation of the silicone film under these conditions. Thevarious silicones employed are described in the results.

Table I It will be noted from the foregoing that I have provided a novelcoating mixture that imparts improved water repellency to the externalsurface of the glass envelope of fluorescent lamps. This advantage isaccomplished by adding a quantity of a silicone resin such as DowCorning type Z920 resin to a dimethylpolysiloxane fluid such as DowCorning DC200 (350 centistokes viscosity grade) in an appropriate ratio.The disadvantages accompanying the prior water repellent film formingmaterials are eliminated. Further, this novel coating material isamenable to use on high speed fluorescent lamp producing machinery.Still further, this novel water repellent film forming material providesa film which is not tacky and therefore does not cause dirt or dust tocling to the lamp surface.

What I claim is:

1. A process for coating the external surface of an electric dischargelamp, the steps comprising: forming a composition consisting of asolvent, dimethylpolysiloxane and a partially alkoxylated siliconeresin, said dimethylpolysiloxane and said silicone resin each comprisingbetween .05 and .4% by volume of said composition and coating theexternal surface of the lamps by rolling at least one completerevolution in said composition.

2. The process according to claim 1 wherein the solvent is a memberselected from the group consisting of petroleum naphthas, acetates,xylene and alcohols.

3. A process for coating the external surface of an electric dischargelamp, the steps comprising: dissolving a Treatment (solvent ContactAngle Alter Exposure To Quartz Ultraviolet Light, degrees isopropyl 1alcohol) 0 2 4 5 7 13 percent by Hours Hours Hours Hours Hours volumeHours 29 Hours 16 Hours 24 Hours Hours 13% D0200 Z920 .4% D 0200--table, each of the silicone resins (Z920) and the dimethylpolysiloxane(DC200) tends to break down rather rapidly when alone, but when As isapparent from the used together the contact angle (that is, the degreeof water repellency) is substantially greater.

The solvent for the silicone resin and the dimethylpolysiloxane may beselected from solvents such as petroleum naphthas, acetates, xylene oralcohols, the alcohols being for example methyl, ethyl, propyl andisopropyl. The solvent should evaporate readily and be miscible with thesilicone resin. If continuous agitation is employed, a solvent may beutilized which gives an emulsoid.

It has been determined that the film forming materials may be variedover the range of 0.05 to 0.4 ml. of each water repellent material per100 rnls. of solvent; that is, .05 to .4% by volume. Going below thislower limit reduces the water repelling While going above the upperlimit yields a tacky film. Optimum concentrations for improved waterrepellency, a nonetacky film and economy were found to be 0.075% resinand 0.1% dimethylpolysiloxane.

0 partially alkoxylated silicone resin with the alkoxyl groupspredominantly methoxyl and dimethylpolysiloxane in a minor amount ofisopropyl acetate solvent, mixing this mixture with a sufiicient volumeof isopropyl alcohol to produce a mixture wherein each of thedimetllylpolysiloxane and the silicone resin is in a volume ratio of .05to .4% with the solvent, then coating the external surface of the lampsby rolling at least one revolution in the mixture.

References Cited in the file of this patent

1. A PROCESS FOR COATING THE EXTERNAL SURFACE OF AN ELECTRIC DISCHARGELAMP, THE STEPS COMPRISING: FORMING A COMPOSITION CONSISTING OF ASOLVENT, DIMETHYLPOLYSILOXANE AND A PARTIALLY ALKOXYLATED SILICONERESIN, SAID DIMETHYLPOLYSILOXANE AND SAID SILICONE RESIN EACH COMPRISINGBETWEEN .05% AND .4% BY VOLUME OF SAID COMPOSITION AND COATING THEEXTERNAL SURFACE OF THE LAMPS BY ROLLING AT LEAST ONE COMPLETEREVOLUTION IN SAID COMPOSITION.